Process for preparing 1,2-dichloroethane by direct chlorination

ABSTRACT

The preparation of 1,2-dichloroethane (EDC) is carried out by feeding ethylene and chlorine into circulating EDC (direct chlorination) such that the reaction mixture boils and the heat of the reaction is led away from the gas space.

This application is a 371 of PCT/EP97/03399 filed Jun. 30, 1997

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for preparing 1,2-dichloroethane bydirect chlorination.

2. The Prior Art

The preparation of 1,2-dichloroethane (EDC below) by reacting ethylenewith chlorine, which is generally referred to as direct chlorination,takes place with the liberation of heat of reaction. For better controlof the reaction and for dissipating the heat of reaction it is common touse circulating liquid EDC. For this purpose liquid reaction mixture orcrude EDC is taken off from the reaction chamber and the heat ofreaction is utilized by way of a heat exchanger to operate distillationcolumns, for example. Such processes are known, for example, fromEP-A-471 987 (ZA 91/6491), DE-A-4029314 and DE-A-41 33 810. From thesedocuments it is also known that particularly intensive mixing of thereactants with the circulating EDC can be ensured by means ofappropriate devices such as static mixers U.S. Pat. No. 4,873,384describes a process for preparing EDC from ethylene and chlorine inliquid EDC in which the vapor of the reaction medium serves to recoversome of the latent heat. U.S. Pat. No. 4,873,384 describes a process forpreparing EDC from ethylene and chlorine in liquid EDC in which thevapor of the reaction medium serves to recover some of the latent heat.

SUMMARY OF THE INVENTION

The invention now relates to a process for preparing EDC by feedingethylene and chlorine into circulating EDC with intensive mixing andheat recovery, which comprises carrying out the reaction at from 65 to125° C. and at from 0.5 to 3.2 bar absolute, the pressure andtemperature being chosen such that the reaction mixture boils, andconducting the heat of reaction away from the gas space and supplying itto a heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for carrying out the process of the invention;

FIG. 2 shows another embodiment of the apparatus of the invention; and

FIG. 3 shows a further embodiment of the apparatus of the invention.

The invention additionally relates to an apparatus for carrying out theprocess, which is shown diagrammatically in FIG. 1. In this figure thereference numbers have the following meanings:

1=Rector

2=Mixing device

3=Limit of the liquid EDC

4=Circulation line for liquid EDC

5=Pump

6=Infeed point for chlorine or ethylene

7=Infeed point for chlorine or ethylene

8=Offtake line for gaseous reaction mixture

9=Line to the heat exchanger 10

10=Heat exchanger

11=Return line from the heat exchanger 10 to the reactor 1

12=Line to the distillation column (not shown)

13=Line to or from the heat consumer unit

14=Line to or from the heat consumer unit

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the novel process and of the apparatus aredescribed in more detail below:

One process variant consists in taking off gaseous reaction mixture fromthe gas space, condensing the EDC in a heat exchanger and passing theliquid EDC back into the reactor.

Another embodiment of the invention consists in feeding the gaseousreaction mixture at the side into a distillation column from which inertgas fractions and unreacted ethylene are taken off from the top, pureEDC is taken off at the side below the infeed point, and high-boilingbyproducts are separated off from the bottom. This distillation columncan advantageously be operated with the heat of reaction from the gasspace of the reactor. In this case the temperature in the lower part ofthe distillation column is somewhat lower than the temperature in thereaction chamber. It is, for example, 90° C. if the reaction is carriedout at 105° C.

An appropriate apparatus for this embodiment of the invention is shownin FIG. 2. In this figure the reference numerals 1 to 14 have themeanings given above, and the others are:

15=Distillation column

16=Line for volatile products

17=Condenser

18=Circuit line

19=Return flow vessel

20=Pump

21=Line for taking off low-boiling products

22=Drier

23=Line for off-gas

24=Condenser

25=Pump

26=Line for EDC

27=Line for high-boiling products

The volatile products pass from the head of the distillation column 15through the line 16 and the condenser 17, by way of the circuit line 18,into the container 19 (return flow vessel). In addition, condensedliquid products pass via the circuit line 18 and a pump 20 into a drier22, which prevents entrained water from becoming enriched in thiscircuit and causing corrosion. Via a line 21 it is possible to bring outlow-boiling products separately.

Gaseous products, essentially unreacted ethylene and inert fractions,pass from the container 19 via a further condenser 24 and a pump 25 tothe off-gas utilization unit.

The drier 22 can be of customary design and may function, for example,in accordance with known physical and/or chemical methods. If the drier22 contains a drying agent, chemical drying agents such as phosphoruspentoxide or physical drying agents such as molecular sieves or silicagels are appropriate. Drying is advantageously effected as indicated inU.S. Pat. No. 5,507,920.

In a different embodiment of the invention the distillation column isoperated under reduced pressure. This embodiment is shown in FIG. 3. Inthis figure the reference numbers 1 to 21 (there is no drier 22) and 23to 27 have the meanings given above and 28 is a return flow line fromthe condenser 24 to the container 19.

In this case the container 19 is under a more greatly reduced pressurethan the column 15 (for example 0.8 bar absolute in the column 15, 0.26bar absolute in the container 19). Pressure regulation here is by meansof one or more pumps, for example the pump 25 (with appropriate valves,which are not shown in the figure). In this embodiment the productswhich arrive by way of the condenser 17 are depressurized in thecontainer 19. The gas phase passes via the line 23 into the condenser24, from which liquefied products flow back to the container 19 via theline 28. The liquid phase—pure EDC—is separated downstream of the pump20 into the product stream (via line 26) and the return stream 18.

The process is carried out with the customary catalysts. Suitablecatalysts are combinations of Lewis acids such as iron(III) chloride andhalides of metals of the first or second subgroup of the Periodic Tableof the Elements, especially sodium chloride, in a wide variety of molarratios (NL-A-6901398, U.S. Pat. No. 4,774,373 or DE-A-41 03 281) and, inparticular, with the catalyst system according to WO-A-94/17019 (ZA94/0535), in which case during the entire reaction the molar ratio ofsodium chloride to iron(III) chloride remains below 0.5, preferably inthe range from 0.45 to 0.3. In this process the EDC is obtained in suchhigh purity that particularly long standing times of the heat exchangersare achieved.

The novel implementation of the process entails a range of advantages:

The reaction can be carried out very safely and can be readilycontrolled at any time. By this means it is possible to keep thereaction temperature low, which suppresses the formation of byproducts.Owing to the fact that the heat of reaction is conducted away from thegaseous reaction mixture, the heat exchangers, for example circulationevaporators, can be given small dimensions, since the heat ofcondensation of the EDC is utilized as well. Another advantage is thatthe heat exchangers are not contaminated by entrained catalyst andhigh-boiling byproducts.

The utilization of the heat of reaction and heat of condensation is veryeffective and permits a large number of constructional designs of theprocess. The heat exchanger or exchangers can be arranged directlyadjacent to the reactor, and the heat-utilizing apparatus can in turnalso be built in the direct spatial vicinity of or around the heatexchanger or exchangers. By this means it is possible to avoidconstructional expense and heat losses as a result of long lines and tosave valuable space in the plant.

In the case of the abovementioned embodiments of the invention, in whichinert gas fractions and unreacted ethylene are removed, the ethylene canbe separated off from the inert fractions in a known manner and passedback to the process. Gas fractions such as oxygen or nitrogen are, forexample, entrained by the chlorine, the oxygen here being regarded asinert as it is at a volume concentration below the explosion limit (3%).Off-gas recycling in the context of direct chlorination is described inWO-A-96/03361 (ZA 95/6058).

The implementation of the reaction is effected in a manner known per se,reference being made to the abovementioned documents in relation to thisand to the details regarding apparatus.

The novel process is explained in more detail in the following examples.

EXAMPLE 1 (FIGS. 1 and 2)

In a direct chlorination reactor 1 with a static mixer 2, chlorine isfed in via the line 6 and ethylene is fed in via the line 7. The reactoris filled with liquid EDC to the liquid level 3 and this EDC is pumpedin circulation via the line 4 and the pump 5. The gas mixture whichemerges from the vapor space of the reactor via the line 8 (essentiallycomprising EDC but also traces of unreacted ethylene, oxygen, nitrogenand components which boil more readily than EDC) is predominantly (about85% ) passed via the line 9 to a column heater 10 (heat exchanger),where it is condensed and passed back into the reactor 1 via the line11. The energy of condensation is passed via the lines 13 and 14 to thedistillation column 15 and is led away from the latter.

The smaller proportion of the gas mixture is fed via the line 12 intothe distillation column 15 at the side, where unreacted ethylene,oxygen, nitrogen and traces of relatively low-boiling byproducts such asethyl chloride and water are separated off at the top (line 16). Thepure EDC is taken off from the column 15 via the line 26 (below theinfeed point of the line 12).

Noncondensables such as ethylene, oxygen and nitrogen pass via the line16, the condenser 17, the line 18, the container 19 and the line 23 toan off-gas condenser 24, and then to the compressor 25, which sends themunder pressure to an off-gas utilization unit.

Condensables such as relatively low-boiling byproducts and an azeotropicmixture of EDC and water likewise pass first via the line 16, thecondenser 17 and the circuit line 18 to the return flow vessel 19, butfrom there they pass via the conveying pump 20 to the drier 22, whichprevents traces of entrained water from accumulating at the column headThe dried condensate then flows via the circuit line 18 into thedistillation column 15.

EXAMPLE 2 (FIG. 3)

The procedure of Example 1, first paragraph, is repeated, and then theprocedure is as follows:

The smaller part of the gas mixture is fed via the line 12 into thedistillation column 15. Pure EDC and unreacted ethylene, oxygen andnitrogen and traces of relatively low-boiling components pass via theline 16 to the EDC condenser 17 and then via the line 18 to the returnflow vessel 19. A (single) vacuum pump 25 is used to establish apressure of 0.8 bar absolute in the column 15 and 0.26 bar absolute inthe return flow vessel 19, in order to separate off the unreactedethylene dissolved in the EDC, and also the oxygen and nitrogen. FurtherEDC is condensed at +1° C. in the off-gas condenser 24, and the off-gasis passed via the line 23 to an off-gas utilization unit. Pure EDC fromthe return flow vessel 19 is passed via the line 26 to an EDC crackingfurnace.

What is claimed is:
 1. A process for preparing 1,2-dichloroethane (EDC)by feeding ethylene and chlorine into circulating EDC with intensivemixing and heat recovery, which comprises carrying out reaction in areactor (1) at a temperature of from 65 to 125° C. and at a pressure offrom 0.5 to 3.2 bar absolute, pressure and temperature being chosen suchthat a reaction mixture boils; conducting heat of reaction away from agas space and supplying said heat to at least one heat exchanger (10);taking off part of the gaseous reaction mixture from the gas space,condensing the EDC in said heat exchanger (10) and passing the liquidEDC back to the reactor; feeding a part (12) of the gaseous reactionmixture (8) at a side infeed point into a distillation column (15) fromwhich inert gas fractions and unreacted ethylene (16) are taken off fromthe top, pure EDC (26) is taken off at the side below the infeed point,and high-boiling by products (27) are separated off from the bottom; andproviding from said distillation column (15) said line for volatileproducts (16) to a condenser (17) having a circuit line (18) to a returnflow vessel (19) connected to a pump (20) having a line for taking offlow-boiling products (21) and a drier (22) and there being a line foroff-gas (23) leading to a condenser (24) and a pump (25).
 2. The processas claimed in claim 1, wherein the distillation column is operated withthe heat of reaction from the gas space of the reactor.
 3. The processas claimed in claim 1, wherein the intensive mixing is effected if witha static mixer.
 4. The process as claimed in claim 1, wherein thereaction is carried out with a catalyst system comprising a Lewis acidand a halide from the first or second group of the Periodic Table of theElements.
 5. The process as claimed in claim 4, wherein the catalystemployed comprises sodium chloride and iron(III) chloride in a molarratio of below 0.5.
 6. An apparatus for carrying out a process forpreparing 1,2-dichloroethane (EDC) by feeding ethylene and chlorine intocirculating EDC with intensive mixing and heat recovery, comprising areactor (1), a mixing device (2), a limit of the liquid EDC (3), acirculation line for liquid EDC (4), a pump (5), infeed points forchlorine and ethylene respectively (6, 7), an offtake line for gaseousreaction mixture (8), a line (9) to a heat exchanger (10), a return line(11) from the heat exchanger (10) to the reactor (1), a line (12) to adistillation column (15) and lines (13, 14) to and from, respectively,the distillation column (15); and said distillation column (15) having aline for volatile products (16) leading to a condenser (17) and acircuit line (18) leading to a return flow vessel (19) and a Dump (20)having a line for taking off low-boiling Products (21) with a drier(22), a line for off-gas (23) leading to a condenser (24), a pump (25)with a line for EDC (26) and a line for high- boiling products (27). 7.The apparatus as claimed in claim 6, in which the drier (22) is omittedand a return flow line (28) from the condenser (24) to the container(19) is provided.